Operations performed on the bones of young patients often present a common drawback, namely that of subsequently limiting or blocking growth of such bones.
For example, in the event of a tumor being resected at the knee, it is known to replace a portion of the bone and its joint with a prosthesis. Unfortunately, it is the portion of the bone that is situated close to the knee joint that presents the greatest potential for growth. Under such circumstances, the patient's operated leg will lengthen less than the other leg, thereby giving rise to unbalance.
The solution presently used for remedying that problem consists in regularly re-operating the patient so as to lengthen the prosthesis. It can be understood that that solution is not very satisfactory since repeated operations are correspondingly traumatic for the patient, and they also make the patient run risks.
Furthermore, in the event of a major deviation of the vertebral column (scoliosis or kyphosis), it is known to use rigid link rods that serve to straighten the vertebral column. For that purpose, the link rods are curved so as to have appropriate curvature, and then they are blocked parallel to the vertebral column by means of a plurality of hooks and screws fastened to certain vertebrae of the vertebral column. There is then a risk of the link rods preventing any growth of the vertebral column, thus requiring repeated operations every six months in order to lengthen the rods and allow the child's vertebral column to continue growing. Such repeated operations include risks of frequent complications such as vertebrae fusing together, thereby preventing any mobility in the vertebral column.
A known solution for accompanying the lengthening of the vertebral column without a new operation is described in Document US 2009/0204156.
It consists in connecting two parallel rods by a mechanism that leaves the rods free to move in translation when they are pulled apart from each other but that block movement of the two rods when they are pushed towards each other. The implant is thus free to lengthen as the patient grows, but it cannot shorten, so it continues to perform its function of straightening the vertebral column.
The mechanism described in that document comprises a body having two parallel housings passing therethrough, each of which receives one of the two rods. Each housing opens out at one end of the body via a large mouth and presents a section that tapers progressively towards the other end of the body.
A split ring is then mounted on each of the rods and is held inside each housing of the body by a plug. More precisely, each plug is cut so as to form a spring that urges each split ring to press against the narrowing section of each housing of the body.
Thus, when the rods are pushed towards each other, they push the split rings into the narrowing sections of the housings in the body, such that the split rings become compressed onto the rods and block them.
In contrast, when the rods are pulled apart from each other, they pull the split rings away from the narrowing sections in the housings, so that the split rings can expand and release the rods.
The major drawback of that mechanism is its poor reliability over time.
The resilient characteristics of the plugs run the risk of changing over time, so that the implant can no longer perform the function for which it is designed.
An effect of the split rings adhering to the rods is also to be feared, which would make the mechanisms inoperative.